In spite of the success of an attenuated measles virus (MV) vaccine in the modern world [1] measles virus (MV) is still a major killer of children in developing countries [2]. MV strikes an estimated 20 million children a year and killed around 164,000 individuals in 2008 according to the World Health Organization. MV causes an acute disease characterized by fever, photophobia, coughing, running nose, nausea, and a macular red rash over most of the body. In rare instances, persistent MV infections can occur in the brain and lead to encephalitis. Humans and monkeys are hosts for MV [3,4,5,6,7] while most rodents are not normally infected by the virus [8,9,10]. The recent discovery that attenuated strains of MV possess oncolytic properties and can be used to destroy tumor cells, has kindled an interest in this virus as a gene therapy agent [11,12].
Measles virions contain a negative strand RNA genome from which viral mRNA's are transcribed to encode a nucleocapsid protein (NP), a phosphoprotein (P), virulence factors (C and V), matrix protein (M), membrane fusion protein (F), the hemagglutinin/receptor binding protein (H), and an RNA polymerase (L) [13]. Surrounding the nucleocapsid is a membrane which contains the two viral glycoproteins, H and F. The H protein is required for viral attachment to the host cell receptor, while F mediates membrane fusion and entry at the host plasma membrane and is also responsible for syncytia (multi-nucleated cell) formation.
Interaction of the H protein of MV with a cellular attachment factor is the initial event of infection. The binding of H to the host cell receptor triggers and activates the F protein to induce fusion between virus and host cell membranes [14,15,16]. The search for MV cellular receptors initially began with vaccine/laboratory strains and progressed to more relevant receptors used by wild type MV (wtMV) isolates [17]. Human membrane cofactor protein (MCP/CD46) is a receptor for the Edmonston laboratory/vaccine strain of MV [18,19]. CD46 is a complement regulatory protein that is expressed on most cell types in the human body, with the exception of red blood cells (although it is on monkey erythrocytes) [20]. Natural isolates of wtMV can be adapted to grow in Vero monkey kidney cells and this is accompanied by mutations in the H protein that convey the CD46 receptor binding phenotype [21,22,23]. Strains of wtMV are routinely isolated in marmoset B95-8 cells, a B cell line immortalized with Epstein-Barr virus, which allows the virus to grow without the need for adaptation [24]. These isolates cannot use CD46 as a receptor [22,25]. Our laboratory and others hypothesized that another lymphotropic receptor could be used by wild type isolates of MV [22,26,27]. Signaling lymphocyte activation molecule (SLAM) or CD150 was identified to be a lymphotropic receptor for both clinical isolates and vaccine strains of MV [28,29,30]. SLAM/CD150 is a signaling molecule that is expressed on activated B, T, monocyte, and dendritic cells [31].
Recent evidence indicates that CD150+ alveolar macrophages, dendritic cells, and lymphocytes are the initial targets for measles virus infections in macaques [32,33,34,35]. However, wild type MV, in autopsied human patients and some experimentally infected monkeys, has been shown to infect the epithelial cells of the trachea, bronchial tubes, lungs, oral cavity, pharynx, esophagus, intestines, liver, and bladder [36,37]. These epithelial cells do not express SLAM/CD150, but the infected cells do shed virus [37,38,39]. Epithelial cells may be important later on in infection and for the spread of MV by aerosol droplets. Wild type MV does not readily infect most common laboratory epithelial, endothelial, or fibroblast cell lines. In addition, cryo-preserved primary human small airway epithelial cells (SAEC) grown in serum free epithelial cell growth medium are not normally susceptible to wtMV, but can be made susceptible by culturing them in 2% fetal calf serum [39]. These cells do not express CD150/SLAM and the wtMV cannot use CD46/MCP, suggesting that there is another receptor on epithelial cells [39]. Other investigators have been searching for an elusive receptor on polarized epithelial and cancer cell lines [41,44,45,46].
Herein it is shown that wild type measles virus infects primary airway epithelial cells grown in fetal calf serum and many adenocarcinoma cell lines of the lung, breast, and colon. A microarray analysis of permissive versus non-permissive cell lines showed that transcripts for many adherens junction and tight junction proteins were up-regulated in virus susceptible cells. However, the integrity of these junctions was not a prerequisite for infection. Non-permissive cell lines could be infected following transfection with a CD150/SLAM expression vector, indicating that they were replication competent. Analysis of the microarray data, filtered for membrane protein genes, produced a short list of 11 candidate receptors. Of these only human PVRL4 (Nectin 4), a tumor cell marker found on breast, lung, and ovarian carcinomas, rendered cells permissive to measles virus infections. Antibodies directed against PVRL4 or PVRL4 siRNA's abolished wtMV infection.